TFTs are three terminal electronic devices which, for display applications, are typically fabricated in large numbers upon an underlying insulating substrate as a regular two dimensional array organized as rows and columns. Two of the three terminals of each TFT are each typically connected to a respective row and column metalization line which are also disposed on the substrate. The third terminal, or pel electrode, is an electrically "floating" terminal relative to the first and second terminals. In order to complete an electrical circuit of the TFT through the floating pel electrode, subsequent processing steps are required. These processing steps, particularly in the fabrication of a flat panel display type of device, involve coplanarly mounting at some distance away from the array a glass panel having a transparent conductive coating. Such a glass panel comprises, in the simplest case, a counter electrode uniformly disposed over a surface serving as the common counter electrode for the underlying TFT pel electrodes. The space between the underlying substrate and the overlying plate is filled with a suitable liquid crystalline material. Circuit continuity is thereby provided through the intervening liquid crystal (LC) material from the pel electrode to the counter electrode, such that when the TFT device is suitably energized, the intervening liquid crystal material is activated, thereby forming a single pixel of the flat panel display.
In modern flat panel displays such an array of TFT structures may comprise from ten thousand up to a million devices. For example, to construct a medium resolution color display one million TFT devices may be required. As can be appreciated, the testing of such a structure presents a formidable manufacturing problem. This problem is compounded by the fact that a TFT device is not operable and, hence, cannot be tested until the flat panel display is completely fabricated. That is, both the overlying transparent plate having the counter electrode and the intervening liquid crystal material must be in place. As may be further appreciated, any defects in the array of TFT devices will therefore not be detected until after the final assembly of the display. Thus, for those arrays of devices that are defective for one reason or the other, such as having open, shorted, or functioning but out of electrical specification devices, the added expense of this final fabrication procedure must be incurred in order to determine the nonfunctionality of these devices.
Although it is conceivable that a mechanical type of probe may be employed to individually test the TFT devices by contacting the pel electrode of each device, this approach is not economically feasible. Any such mechanical probe need be of a very fine geometry in order to probe the relatively small pel electrode structures. The use of such a mechanical probe also implies that either the probe or the substrate be linearly translated in a precise manner during the testing operation, resulting in an overall test time for a large array which would prove to be economically unjustifiable.